In arsenic-cadmium combined pollution soil, about one-sixth of the paddy fields suffer from arsenic-cadmium combined pollution in different degree, and the pollution area is increasing year by year. Mining and smelting is the main way of arsenic-cadmium combined pollution in soil; waste slags and tailing sands usually contain high concentration arsenic and cadmium waste ores or tailing sands; after natural weathering and rain erosion, arsenic and cadmium will be released to soil, so that the soil suffers severe arsenic-cadmium combined pollution. Too high cadmium and arsenic can cause the gradual variation of the soil biological characteristics, resulting in decreased soil quality. Also, arsenic and cadmium in the soil have strong biological migration and toxicity, extremely easily absorbed by crops and accumulated, and directly affects the crop quality and yield. Moreover, they have a serious harm to human health through the food chain. Therefore, how to repair arsenic-cadmium combined pollution soil has become a major problem to be solved.
In recent years, the application of the biochar material in heavy metal pollution remediation gets attention gradually. The biogenic carbon material has a wide range of sources, has a large number of microporous structures and a large specific surface area, strong adsorption capacity, and its adsorption behaviour can affect the migration, transformation, bio-ecological effects of heavy metals in the environment and contaminated environmental media control and repair and other processes. The biochar contains, on the surface, a large number of functional groups, such as carboxyls, hydroxyls, anhydrides, etc., as well as negative charges, and has a larger surface area. After having been applied to the soil, the biochar can adsorb heavy metals and immobilize them on the surface, which can significantly reduce the bioavailability of most heavy metals and improve the physical, chemical and biological properties of the soil, such that soil fertility and crop yield are both improved to some extent.
However, the biochar material can significantly improve the mobility and availability of arsenic in soil. It has been reported that the biochar can reduce the concentration of cadmium and zinc in the soil filtrate (by 300 times and 45 times, respectively), but the concentration of arsenic in the filtrate is obviously improved and the biochar improves the mobility of arsenic. In addition, it was reported that the addition of the biochar could improve the microenvironment of rice rhizosphere and promote the formation of a root surface iron film; as a result, the concentrations of Cd, Zn and Pb in rice roots are decreased by 98%, 83% and 72%, respectively, but the arsenic concentration is increased by 327%. Therefore, how to improve the composition and properties of the biochar and improve the capacity of adsorption and fixation of arsenic thereof, and at the same time to reduce the effectiveness of arsenic and cadmium in the soil is very challenging and also has a great environmental significance.
It has been reported in a number of literatures that the zero-valent iron can reduce the bioavailability of arsenic in the soil, and the zero-valent iron has strong electron transfer and adsorption ability; under the aerobic conditions, the biomineralization and iron oxidation are promoted by the electron transfer, thereby promoting the oxidation of trivalent arsenic and the adsorption fixation of the pentavalent arsenic in the soil; under the anaerobic conditions, the trivalent arsenic may be directly reduced to zero. The bioavailability of arsenic depends on its valence and morphology, and the promotion of the oxidation and adsorption of arsenic in the soil can reduce its effectiveness. Therefore, how to combine the biochar with the environmental effect of zero-valent iron to prepare an iron-based biochar composite material and at the same time reduce the effectiveness of arsenic and cadmium in soil is of great significance.